An automobile equipped with an anti-skid brake system is provided with means for detecting the wheel speed through the rotation of a bearing supporting each vehicle wheel.
One of such revolving speed detecting devices is disclosed in Japanese Patent Application 3-247766 filed by the present applicant.
This device comprises, as shown in FIGS. 5A and FIG. 5B, a pulser ring 33 and a detector ring 34 having, respectively, a plurality of teeth 35 and 36 and mounted on the respective peripheral surfaces of a rotatable bearing ring 31 and a fixed bearing ring 32. Around the detector ring 34 is wound a coil L connected to a high-frequency voltage application means 37. In this arrangement, by exciting the coil L with a high-frequency voltage having a frequency fo, a magnetic circuit is formed between the pulser ring 33 and the detector ring 34. When the pulser ring 33 begins to rotate in this state, the magnetic reluctance will vary, which will in turn vary the inductance of the coil. The output voltage of the coil is applied to a rectifying/smoothing circuit 38 to remove carrier components of fo. The signal thus obtained is compared with a set value m in a comparison circuit 39 to obtain a pulse signal corresponding to the revolving speed of the bearing.
Because of its complicated structure, the detector ring 34 has to be made by forming soft iron. But, since such a metal has a high conductivity, an eddy current will be produced if such a ring is placed in an alternating magnetic field. If an eddy current is produced in the detector ring 34, its sensitivity to the inductance of the coil will drop, thus making it difficult to detect the rotation with a high degree of accuracy.
Further, the above device uses a fixed value m as the set value to be compared with the smoothed signal in the comparison circuit 39. Actually, however, the output voltage of the coil L may change due to dimensional variations when assembling the circuits or fluctuations in temperature while in use and as a result, the amplitude or the average of the smoothened voltage signal may fluctuate markedly.
In such a case, no pulse signal indicative of the revolving speed is obtainable unless the fixed set value m is within the range of fluctuation of the smoothed signal.